This invention relates to a method for purifying silicon, and more particularly to a method whereby metallurgical grade silicon can be sequentially purified and the purified silicon used to produce a high quality monocrystalline silicon ingot.
The semiconductor industry uses large and ever increasing quantities of high quality monocrystalline silicon. This is especially true with the growing interest in silicon solar cells and with the large volume of silicon that these cells require. This material must be of high crystalline quality, low in impurity content, and of a prescribed resistivity. Such material is often described by the term "semiconductor grade" silicon. To make the solar cells a competitive source for power generation, there must be a significant reduction in the price of this semiconductor grade silicon.
Metallurgical grade silicon is an impure form of silicon formed by the arc reduction of silica sand. It is electrically similar to a poor conductor rather than to a good semiconductor. The metallurgical grade silicon contains about one to two percent impurities including iron, aluminum, calcium, chromium, manganese, nickel, titanium, and vanadium. It also contains small quantities of boron, phosphorus, and other impurities. Most high quality silicon is produced today by converting the metallurgical grade silicon into liquid and gaseous halosilanes which can be purified, for example, by distillation. The purified halosilanes can be reduced with hydrogen to deposit pure polycrystalline silicon on a heated filament. This pure polycrystalline silicon is then used to charge a crystal puller and a monocrystalline ingot is pulled by the Czochralski technique. Such a process is expensive and is not likely to lead to low enough costs to be feasible for solar cells in large scale applications.
Accordingly, it is an object of this invention to provide an improved process for the purification of silicon.
It is a further object of this invention to provide a process for the purification of metallurgical grade silicon and for the conversion of that purified silicon into monocrystalline silicon ingots.
It is another object of this invention to provide a process whereby impure silicon can be purified and converted into semiconductor grade monocrystalline silicon.
It is a still further object of this invention to provide a low cost, continuous process for converting impure silicon to pure and dislocation-free single crystalline silicon.